The present invention relates to a novel photoacid generator, a method of changing a structure of a resin with said photoacid generator, particularly a photoresist containing said photoacid generator, and a method of cross-linking a resin using such a photoacid generator.
In recent years, industries of Taiwan develop rapidly along with the rise of the information industry, wherein the semiconductor industry bears the brunt of this impact. Along with the rapid development of the semiconductor fabricating processes, the versatility of functions and miniaturization of the products have become the research objectives of the industry. With no doubt, the lithographic techniques are crucial in a semiconductor fabricating process. In brief, the basic steps of the lithography comprise (1) coating a photoresist, (2) exposing, and (3) developing. Firstly, a photoresist is coated on a surface of a chip. Next, a specific light source is irradiated on the photoresist coating through a photomask thereby imagewise exposing the photosensitive material of the photoresist. This step is called xe2x80x9cExposurexe2x80x9d. Next, a suitable developer is used to remove the exposed or unexposed photoresist. This step is called xe2x80x9cDevelopingxe2x80x9d. The change of a structure of a polymer contained in the photoresist prior to and after the exposure causes a change in the solubility of the polymer in the developer, creating a soluble region and an insoluble region according the pattern of the photomask, and thus the pattern transfer is achieved.
In general, according to the change of the solubility of the polymer in the developer, photoresists can be classified into positive tone photoresists and negative tone photoresists according to the following:
Positive tone: After exposure, the exposed region has a higher solubility in the developer than the unexposed region.
Negative tone: After exposure, the exposed region has a lower solubility in the developer than the unexposed region.
After decades of R and D, numerous positive tone or negative tone photoresists have been proposed, which includes the free radical polymerization photoresists of diazo and azide types in the early days and the chemical amplication photoresists predominant in the recent market. The reaction mechanisms of the chemical amplication photoresists have diversified from the initial two mechanisms:
(1) photoacid-catalyzed, epoxide-ring-opening cross-linking reaction; and
(2) photoacid-catalyzed, t-butoxycarbonyl (t-BOC) deprotection; to the following reaction mechanisms:
(3) photoacid-catalyzed dehydration;
(4) photoacid-catalyzed rearrangement;
(5) photoacid-catalyzed condensation;
(6) photoacid-catalyzed ionic polymerization;
(7) photoacid-catalyzed depolymerization.
The fundamental principle of a chemical amplication photoresist is utilizing a photoacid generator (PAG), after irridation, to react with a H-donor (usually a solvent or other material) thereby generating a proton acid. In a post-exposure bake (PEB) process following the irridiation, the proton acid initiates a series of chain-breaking, cross-linking or other chemical reactions of a polymer in the photoresist as an acid catalyst, causing a structural change and a difference in solubility to the developer between the exposed region and the unexposed region of the polymer, so that a positive tone or negative tone pattern are obtained. The use of a chemical amplication photoresist, in addition to greatly increasing the sensitivity of the photoresist, also improves the contrast and resolution of the photoresist pattern.
Generally speaking, chemical amplication photoresists, according to the difference in the quantity of the major components, can be classified into binary photoacid photoresists and ternary photoacid photoresists. A binary photoacid photoresist mainly comprises a photoacid generator and a polymeric resin; while a ternary photoacid photoresist mainly comprises a photoacid generator, a dissolution inhibitor or a cross-linking agent, and a polymeric resin. Take the famous t-BOC deprotection photoresist proposed by H. Ito, et al. In U.S. Pat. No. 4,491,628 as an example. This photoresist belongs to a binary photoacid photoresist containing triphenylsulfonium hexafluoroantimonate as the photoacid generator and poly(4-t-butoxycarbonyloxystyrene) (PBOCST) as the polymeric matrix, the reaction mechanism of which is shown in the following Scheme 1: 
In Scheme 1, triphenylsulfonium hexafluoroantimonate, upon subjected to UR irradiation, releases a proton acid. The PBOCST, which has a weaker polarity, releases t-butoxycarbonyl (abbreviated as t-BOC) in the presence of an acid catalyst, and forms poly(4-hydroxystyrene) (abbreviated as PHOST) having a higher polarity. In other word, there is a conspicuous difference in solubility between the exposed region and the unexposed region. By selecting an appropriate developer, e.g. a non-polar organic solvent, a negative tone photoresist pattern can be obtained. Alternatively, by using a polar aqueous base as the developer, a positive tone photoresist pattern can be obtained. Since PBOCST and PHOST do not have a high absorbency to UV light at 250 nm, this photoresist is suitable for deep UV lithography. Furthermore, PHOST can be dissolved in an aqueous base. Undoubtedly, this photoacid amplication t-BOC deprotection photoresist is a turning point in the development of photoresists.
Other typical photoresists that contain a photoacid generator are disclosed in U.S. Pat. Nos. 5,585,223; 5,532,106; 5,391,465; 5,296,332; and 5,055,439.
Photoresists not only can be applied in the semiconductor industry, but also have unique applications in other fields. For example, in a photoacid-catalyzed epoxide ring-opening reaction (as shown in Scheme 2) or in a photoacid-catalyzed ionic polymerization reaction, a photoresist can greatly increase the molecular weight of the polymer and receives wide applications in the manufacture of microelectromechanical devices (MEMs). A high level of cross-linking is necessary for forming a pattern with a high aspect ratio in a thick film ( greater than 50 micron) by lithography, wherein said aspect ratio is a ratio between the film thickness and the resolution. 
The photoacid-catalyzed dehydration reaction, generally speaking, can be classified into the intermolecular dehydration and the intramolecular dehydration. The reaction mechanism of the intermolecular dehydration is shown in Scheme 3, wherein the photoacid-catalyzed dehydration will cause cross-linking of the polymeric resin of a negative tone photoresist. The intramolecular dehydration reaction (as shown in Scheme 4) is similar to a photoacid-catalyzed reforming reaction (as shown in Scheme 5), both of which will alter the polarity of the polymeric resin. As a result, a polar solvent or a nonpolar solvent can be selected as a developer to obtain a positive tone or a negative tone photoresist pattern. 
One objective of the present invention is to provide a novel photoacid generator.
Another objective of the present invention is to provide a photoresist.
Still another objective of the present invention is to provide a method of carrying out a photoacid-catalyzed reaction in a resin system.
Still another objective of the present invention is to provide a method of photo-cross-linking a resin system.
In order to achieve the above-mentioned objectives, a photoacid generator synthesized according to the present invention has the following structure of formula (I): 
wherein Rxe2x80x2 and R are radicals which enable the photoacid generator (I) forming 
and RH under an irradiation, wherein Rxe2x80x2 is defined as above, and RH is a proton acid.
Preferably, R in the formula (I) is a halogen, 
Preferably, Rxe2x80x2 in the formula (I) is hydrogen, methyl or chloromethyl.
Preferably, Rxe2x80x2 is hydrogen, when R in the formula (I) is not a halogen.
Preferably, Rxe2x80x2 is not hydrogen, when R in the formula (I) is halogen.
Preferably, the photocaid generator (I) is 1,4-dichloromethyl-2-nitrobenzene, 2-nitrobenzyl ester of formic acid or 2-nitrobenzyl ester of acetic acid.
The present invention in one aspect also discloses a method of photo-catalyzing a reaction in a resin system with the photoacid generator (I), which comprises subjecting said resin system with an irradiation in the presence of said photoacid generator (I).
Preferably, the reaction undergoing in said resin system is selected from the group consisting of (1) a photoacid-catalyzed, epoxide-ring-opening cross-linking reaction; (2) a photoacid-catalyzed, t-butoxycarbonyl (t-BOC) deprotection reaction; (3) a photoacid-catalyzed dehydration; (4) a photoacid-catalyzed rearrangement; (5) a photoacid-catalyzed condensation; (6) a photoacid-catalyzed ionic polymerization; and (7) a photoacid-catalyzed depolymerization.
Said resin system comprises one or more resins, and one or more components that are required in undergoing said reaction, e.g. a cross-linking agent, a cross-linking promoter and a solvent.
Another aspect of the present invention is a photoresist composition comprising the photoacid generator (I); and a polymer.
Preferably, said photoresist composition further comprises an organic solvent.
The term xe2x80x9cpolymerxe2x80x9d used in the present invention may refer to a homopolymer, a copolymer, or an oligomer.
Still another aspect of the present invention is a method of forming a photoresist pattern on a substrate comprising the following steps:
a) coating the above-mentioned photoresist composition on a substrate;
b) imagewise exposing the resulting film in step a) to an irradiation in a pattern;
c) baking said imagewise exposed film; and
d) developing said baked film with a developer thereby forming a photoresist pattern on said substrate.
Preferably, said polymer is a homopolymer having a side chain containing a carboxyl group.
Preferably, said polymer is a copolymer having a side chain containing a carboxyl group.
Preferably, said polymer is a homopolymer having a side chain that will form a carboxyl group upon subjected to heating and in the presence of a proton acid.
Preferably, said polymer is a copolymer having a side chain that will form a carboxyl group upon heating and in the presence of a proton acid.
Preferably, said homopolymer is poly(acrylic acid) or poly(methacrylic acid).
Preferably, said copolymer is prepared by copolymerizing acrylic acid or methacrylic acid with one or more monomers containing a vinyl unsaturated group, or said copolymer is a carboxyl-containing graft copolymer prepared by reacting a cyclic anhydride with a hydroxyl-containing polymer or copolymer, wherein the cyclic anhydride and hydroxyl group undergo a ring-opening reaction.
Preferably, the side chain of said homopolymer contains an acid anhydride group or t-butoxycarbonyl group.
Preferably, the side chain of said copolymer contains an acid anhydride group or t-butoxycarbonyl group. More preferably, the side chain of said copolymer further contains a carboxyl group.
Preferably, said copolymer has the following structure: 
wherein w and z is an integer greater than 0; x and y are integers equal to or greater than 0.
Preferably, said copolymer has the following structure: 
wherein m, x, y and z are integers equal to or greater than 0; and x and z are not 0 at the same time.
Preferably, said photoacid generator is 0.5 to 30% by weight of said polymer in said photoresist composition.
Preferably, the baking in step c) enables an acid-catalyzed dehydration condensation reaction between two carboxyl groups.
Preferably, the baking in step c) enables an acid-catalyzed deprotection of a t-butyl group.
In still another aspect of the present invention, a method of cross-linking a polymer with a photoacid generator is disclosed, wherein said polymer has a carboxyl-containing side chain, or said polymer has a side chain that will form a carboxyl group in the presence of an acid and under heating. Said method comprises subjecting said polymer to an irradiation in the presence of said photoacid generator and subsequently to heating at 120xc2x0 C.-170xc2x0 C. Preferably, said photoacid generator has a structure of the above-mentioned formula (I).